Professor Abbe's Pamper on the Microscope. 201 
inclination of the rays towards the axis of the instrument, and not 
upon tlie oblique incidence of light on the object.* 
The facts here brought forward show, on the one hand, the 
reality of a special optical quality, directly related with the angular 
aperture of the objective, yet independent of any special perfection 
or amplifying power possessed by it, and also show it to be a 
" resolving " power or capacity of separating minute detail, conform- 
ably with the literal sense of the term employed. On the other 
hand, they show unequivocally that the delineation of images of 
minute details of structure must take place under conditions essen- 
tially different from those under which the contour outlines of 
larger parts are formed. In all cases where a " resolving " power 
of this kind is in operation, the reunion of rays proceeding from 
the several points of the object in the focal plane of the image is 
most certainly not to be accounted an adequate explanation of the 
images of such details of the object, for on such a supposition the 
differences would remain absolutely inexplicable. The result, then, 
of this preliminary study is to give the following form to the 
inquiry, namely, to find out the special causes outside the micro- 
scope which operate in the formation of images of small structural 
details, and then to determine individually the mode and manner 
of their intervention. 
* Vide Wenham, in ' Monthly Microscopical Journal,' April 1, " On a Method 
of obtaining Oblique Vision of Surface of Structure," &c. The optical principle 
enunciated by Mr. Wenham is totally irreconcilable with Professor Abbe's theory 
and experimental investigations. 
(Jb be continued.') 
